Method for detecting arc faults

ABSTRACT

An apparatus and method for detecting arc faults that have reduced susceptibility to nuisance tripping. The apparatus includes a current sensor, an input sense circuit, an arcing sense circuit, a power supply, a tripping (firing) circuit, a processor, and an electromechanical interface. The current sensor monitors a power input comprising an AC current, and provides high frequency components of the. AC current to the input sense circuit. The input sense circuit filters and rectifies the AC signal, and provides the rectified signal to the arcing sense circuit. The arcing sense circuit provides a voltage level accumulated over a predetermined time period, and digital signals indicative of possible electrical arcing occurring during the sampling period, to the processor. The processor measures the voltage level, stores information relating to measured voltages and the digital signals, and processes the stored information using one or more algorithms, thereby determining whether the signals resulted from an arc fault or a nuisance load. In the event the signals resulted from an arc fault, the processor activates the firing circuit to trip the electromechanical interface, thereby interrupting the power output to the load.

CROSS REFERENCE TO RELATED APPLICATIONS

Copending application Ser. No. 10/314,715, filed Dec. 9, 2002 andapplication Ser. No. (Attorney Docket No. A42159) filed of even date andassigned to the assignee of the present invention contain subject mattersimilar to that contained herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

BACKGROUND OF THE INVENTION

The present application relates generally to apparatus and methods fordetecting arc faults, and more specifically to arc fault detectionapparatus and methods that are less susceptible to nuisance tripping.

Arc fault detection apparatus and methods are known that employ amicro-controller to measure voltages associated with a load, and toprocess data representing the voltage measurements to determine thepresence of electrical arcing. For example, a conventional arc faultdetection apparatus may be configured to sense an alternating loadcurrent, to filter and rectify the AC signal, and to provide therectified signal to an integrating capacitor. The conventional arc faultdetection apparatus may then use a micro-controller to take measurementsof the voltage across the integrating capacitor, and to convert thevoltage measurements to digital data for subsequent processing by analgorithm. For example, the algorithm may be employed to analyze themeasured voltage levels corresponding to respective cycles of the linevoltage, and to determine whether the voltage measurements arecharacteristic of electrical arc faults or nuisance loads such as dimmerloads, appliance thermostat switching, drill current transitions, randomline voltage spikes, and/or EMI bursts. In the event the voltagemeasurements are characteristic of an arc fault, the conventional arcfault detection apparatus typically trips a circuit breaker todisconnect the power line from the circuit.

Although the above-described conventional arc fault detection apparatushas been successfully employed to detect and distinguish betweenelectrical arcing and nuisance loads, there is a need for arc faultdetection techniques that have increased reliability. For example, dueto the generally chaotic nature of electrical arcing, arc faultstypically produce varying numbers of arcing events per half cycle of theline voltage. In contrast, nuisance loads, e.g., triac-controlled dimmercircuits, typically produce the same number of arcing events per halfcycle, and may therefore produce arcing events periodically overmultiple half cycles. However, conventional arc fault detectionapparatus often cannot reliably distinguish between periodic andnon-period arcing events, and are therefore prone to nuisance tripping.Moreover, such conventional apparatus frequently have difficultydistinguishing between certain start-up and shutdown conditions andelectrical arcing. In addition, although some loads may produce noisyswitching signals having relatively large voltage levels, such signalsare not necessarily indicative of electrical arcing and may beincorrectly characterized as arc faults by conventional arc faultdetection apparatus.

It would therefore be desirable to have improved arc fault detectionapparatus and methods that avoid the drawbacks of the above-describedconventional apparatus and methods.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus and method fordetecting arc faults are provided that have reduced susceptibility tonuisance tripping. In one embodiment, the arc fault detection apparatusincludes a current sensor, an input sense circuit, an arcing sensecircuit, a power supply, a tripping (firing) circuit, a processing unit,and an electromechanical interface. In one mode of operation, thecurrent sensor monitors a Power Input comprising an alternating current(AC), and provides high frequency components of the AC current to theinput sense circuit. Next, the input sense circuit filters and rectifiesthe AC signal at its input, and provides the rectified signal to thearcing sense circuit. The arcing sense circuit then provides a voltagelevel accumulated over a predetermined sampling period, and one or moredigital signals indicative of possible electrical arcing occurringduring the sampling period, to the processing unit. The processing unitis operative to measure the voltage level, to store information relatingto measured voltages and the digital signals provided thereto, and toprocess the stored information using one or more algorithms, therebydetermining whether the signals resulted from an arc fault or a nuisanceload. In the event the signals resulted from an arc fault, theprocessing unit activates the firing circuit to trip theelectromechanical interface, thereby interrupting the Power Output tothe load.

In the presently disclosed embodiment, the micro-controller executes aplurality of algorithms for distinguishing between arc faults andnuisance loads, including a three cycle algorithm (TCA) operative todetermine voltage fluctuations that cancel disturbances caused bynuisance loads of the type that are repetitive or continuously varying.The TCA may be expressed asTCA=|(|V _([n−1]) −V _([n]) |+|V _([n+1]) −V _([n]) |−|V _([n+1]) −V_([n−1])|)|,in which V[n−1] represents a first voltage measurement corresponding toa first cycle of the line voltage, V[n] represents a second voltagemeasurement corresponding to a second cycle of the line voltage, andV[n+1] represents a third voltage measurement corresponding to a thirdcycle of the line voltage. The plurality of algorithms further include afirst pulse counter algorithm for counting the number of arcing eventsoccurring during each half cycle of the line voltage, a second pulsecounter algorithm for capturing timing information relating to one ormore arcing events, and an arcing event counter algorithm for countingthe number of arcing events included in at least one running sum ofvoltage measurements. By executing one or more algorithms to distinguishbetween arc faults and nuisance loads, the arc fault detection apparatuscan detect arc faults with increased reliability, thereby reducing thesusceptibility of the apparatus to nuisance tripping.

Other features, functions, and aspects of the invention will be evidentfrom the Detailed Description of the Invention that follows.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood with reference to thefollowing Detailed Description of the Invention in conjunction with thedrawings of which:

FIG. 1 a is a block diagram of an arc fault detection apparatusaccording to the present invention;

FIG. 1 b is a schematic diagram illustrating the arc fault detectionapparatus of FIG. 1 a;

FIGS. 2 a-2 b are diagrams illustrating transformer configurations thatmay be employed in the arc fault detection apparatus of FIG. 1 a;

FIGS. 2 c-2 d are views illustrating a transformer included in thetransformer configurations of FIGS. 2 a-2 b;

FIGS. 3 a-3 b are schematic diagrams illustrating sum of logarithmcircuits that may be employed in the arc fault detection apparatus ofFIG. 1 a;

FIG. 3 c-3 d are schematic diagrams illustrating comparator circuitsthat may be employed in the arc fault detection apparatus of FIG. 1 a;

FIG. 4 is a flow diagram illustrating a method of operation including athree cycle algorithm performed by the arc fault detection apparatus ofFIG. 1 a;

FIGS. 5 a-5 b are flow diagrams illustrating pulse counter algorithmsperformed by the arc fault detection apparatus of FIG. 1 a;

FIG. 6 is a table illustrating the mapping of measurement data setsemployed by the pulse counter algorithms of FIGS. 5 a-5 b;

FIG. 7 is a flow diagram illustrating an arcing event counter algorithmperformed by the arc fault detection apparatus of FIG. 1 a;

FIG. 8 is a flow diagram illustrating a method of integrating the outputof a comparator circuit using a digital counter performed by the arcfault detection apparatus of FIG. 1 a; and

FIG. 9 is a flow diagram illustrating a method of operation includingthe pulse counter algorithm of FIGS. 5 a-5 b, the three cycle algorithmof FIG. 4, and the arcing event counter algorithm of FIG. 7 performed bythe arc fault detection apparatus of FIG. 1 a.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a depicts an illustrative embodiment of an arc fault detectionapparatus 100, in accordance with the present invention. In theillustrated embodiment, the apparatus 100 comprises a current sensor101, an input sense circuit 102, an arcing sense circuit 104, a powersupply 106, a tripping (firing) circuit 108, a processing unit 112, andan electromechanical interface 117. In an illustrative mode ofoperation, the current sensor 101 monitors a Power Input via anelectromechanical interface 117, and provides high frequency componentsof the Power Input to the input sense circuit 102. Next, the input sensecircuit 102 filters and rectifies the AC signal at its input, andprovides the rectified signal to the arcing sense circuit 104. Thearcing sense circuit 104 then provides voltage levels and digitalsignals indicative of possible electrical arcing to the processing unit112. Next, the processing unit 112 measures the voltage levels andanalyzes the voltage measurements and the digital signals using one ormore algorithms to determine whether the signals resulted from an arcfault or a nuisance load. In the event the signals resulted from an arcfault, the processing unit 112 activates the firing circuit 108, therebytripping the electromechanical interface 117 to disconnect the PowerOutput from the load. By determining whether the AC signal sensed by theinput sense circuit 102 resulted from electrical arcing or from anuisance load before tripping the electromechanical interface 117, theprocessing unit 112 reduces the susceptibility of the arc faultdetection apparatus 100 to nuisance tripping.

FIG. 1 b depicts an illustrative implementation of the arc faultdetection apparatus 100. In the illustrated embodiment, the currentsensor 101 includes a transformer TR1, which monitors the Power Input bymonitoring an alternating current (AC) i flowing through a loadcoupleable to the Power Output via a load line phase terminal TP9 andload neutral terminals TP10 and TP22. The transformer TR1 is configuredfor magnetically coupling the high frequency components of the ACcurrent i from its primary coil L1 to its secondary coil L2, therebyproviding an AC current I to the input sense circuit 102. In thepresently disclosed embodiment, the arc fault detection apparatus 100 isimplemented on a substrate such as a laminated printed circuit board(PCB) substrate, a ceramic substrate, or any other suitable substrate.Further, the primary coil L1 of the transformer TR1 surrounds thesecondary coil L2 (see FIGS. 2 c-2 d), which has a magnetic axisperpendicular to the substrate. In the preferred embodiment, thesecondary coil L2 of the transformer TR1 provides a relatively weakmagnetic coupling. For example, the weakly coupled transformer TR1 mayprovide a mutual conductance of about 20-50 μH.

As shown in FIG. 1 b, the input sense circuit 102 includes a capacitorC1, resistors R1-R2, and diodes D1-D6. The secondary coil L2 of thetransformer TR1 is connected between the capacitor C1 and the resistorR2. The capacitor C1 is also connected to the resistor R1, and theresistors R1-R2 are connected to ground. The capacitor C1 high passfilters the AC signal provided by the transformer secondary coil L2, andthe resistors R1-R2 provide a ground reference for the secondary coilL2. The cathode of the diode D1 is connected to the capacitor C1 and theresistor R1, the cathode of the diode D2 is connected to the secondarycoil L2 and the resistor R2, and the anodes of the diodes D1-D2 areconnected to ground. The cathode of the diode D1 is also connected tothe anodes of the diodes D3-D4, and the cathode of the diode D2 is alsoconnected to the anodes of the diodes D5-D6. The cathodes of the diodesD4-D5 are connected to ground, and the cathodes of the diodes D3 and D6are connected to a node 114 providing the output of the input sensecircuit 102. The diodes D1-D2 and D4-D5 are configured to form a fullwave rectified bridge, and therefore the output provided at the node 114is a full wave rectified signal. In the preferred embodiment, the diodesD3 and D6 are matched to the diodes D4-D5. Further, the diodes D3-D6 anda capacitor C2 included in the arcing sense circuit 104 form a loggingcircuit, thereby causing the level of the output provided at the node114 to be proportional to the log of the input of the input sensecircuit 102.

In the illustrated embodiment, the arcing sense circuit 104 includes thecapacitor C2, an integrating capacitor C3, resistors R3-R7, anoperational amplifier (op amp) 116, and a diode D7. As shown in FIG. 1a, the capacitor C2 and the resistor R4 are connected between the node114 and ground. Further, the resistor R3 is connected between the node114 and pin 10 of a micro-controller, which performs the functions ofthe processing unit 112 (see FIG. 1 a). The op amp 116 and the resistorsR5-R6 are configured to form a non-inverting amplifier 105. Thecapacitor C2 is connected to the non-inverting input of the op amp 116,and the voltage across the capacitor C2 is buffered and provided to theintegrating capacitor C3 via the diode D7 and the resistor R7. Thecapacitor C3 is connected between pin 9 of the micro-controller 112 andground. The diode D7 is configured to prevent reverse current flow fromthe capacitor C3. Further, the combination of the resistor R7 and thecapacitor C3 forms a low pass filter to filter out high frequency noise.

It is noted that the voltage across the capacitor C2 resets with a decaytime of about (C2)*(R4) seconds. For example, if R4 equals 10 kΩ and thecapacitor C2 equals 1 nf, then the decay time of the capacitor C2 isabout 10 μsec. The arcing sense circuit 104 is configured to convert achange in voltage across the capacitor C2 (ΔV_(C2)) into a pulse havinga width tpw, which may be determined fromG*ΔV _(C2) *e ^((−tpw/C2·R4)) =Vcc/2,  (1)in which “G” is the gain of the op amp 116. Each pulse generated inresponse to a significant di/dt event (“an arcing event”) causes achange in voltage across the capacitor C3 (ΔV_(C3)), which may beexpressed asΔV _(C3)=(Vcc−Vbe−V _(C3))*(1−e ^((−tpw/C3·R7)))−V _(C3).  (2)Accordingly, equations (1)-(2) demonstrate that as the number of arcingevents increases, ΔV_(C3) increases with the log of ΔV_(C2), therebyincreasing the dynamic range of the arc fault detection apparatus 100.

The micro-controller 112 is operative to take measurements of thevoltage V_(C3) across the integrating capacitor C3 at pin 9 of themicro-controller. For example, the micro-controller 112 may comprise aMSP430F1122 micro-controller sold by Texas Instruments Inc. (TI),Dallas, Tex., USA, or any other suitable micro-controller. In oneembodiment, the micro-controller 112 measures the voltage V_(C3) acrossthe capacitor C3 once each half cycle of the line voltage near the linevoltage zero crossing. The measured voltages represent the sum ofvoltages accumulated by the integrating capacitor C3, which is reset viaa simple decay time. Accordingly, a sampling period during which eachmeasurement is made has a duration starting when an arcing event occursand lasting for about a decay time.

In alternative embodiments, the micro-controller 112 measures thevoltage V_(C3) across the integrating capacitor C3 multiple times perhalf cycle of the line voltage. For example, the micro-controller 112may measure the voltage V_(C3) twice each half cycle at times determinedby the absolute value of the line voltage, and reset the capacitor C3 to0 volts following each measurement. Specifically, the micro-controller112 r measures the voltage V_(C3) at times corresponding to twopredetermined regions of each half cycle. The micro-controller 112 thensums these measurements by first resetting the capacitor C3 to 0 voltsat the beginning of each predetermined region, and then measuring thecapacitor voltage at the end of each region. In the preferredembodiment, both voltage measurements per half cycle are made near theline voltage zero crossing, e.g., one measurement is made just beforethe zero crossing, and the other measurement is made just after the zerocrossing.

In the presently disclosed embodiment, pin 9 of the micro-controller 112is connected to an analog to digital converter (ADC) within themicro-controller 112. The ADC converts the analog voltage measurementstaken across the integrating capacitor C3 to digital data, therebyallowing the micro-controller 112 to store the measurement data ininternal memory. Following each measurement, the micro-controller 112shorts pin 9 to ground to prepare the capacitor C3 to integrate currentfor the next sampling period. In addition, pin 10 of themicro-controller 112 is connected to the output of the op amp 116, whichprovides a pulse counter signal directly to pin 13 of themicro-controller 112. The micro-controller 112 employs an internalcounter to monitor the pulse counter signal to keep track of pulsesoccurring within the signal. The micro-controller 112 then stores datarelating to the measured voltages and the monitored pulses, andprocesses the data using one or more algorithms to determine whether thevoltages/pulses were generated by an arcing event or by a nuisance load.

The arc fault detection apparatus 100 further comprises a reset circuit110, which includes a capacitor C4, resistors R8-R10, a zener diode D8,and a pushbutton PB1 operable to connect the line phase to pin 12 of themicro-controller 112. In the illustrated embodiment, the seriallyconnected resistors R9-R10 and the resistor R8 connected between pin 12and ground reduce the line voltage and the line current to levelssuitable for the micro-controller 112. Even though the TI MSP430F1122micro-controller includes internal protection diodes, the zener diode D8is connected between pin 12 and ground to provide a redundant voltagelimitation. The capacitor C4 is connected between pin 12 and ground tofilter out high frequency noise. When the pushbutton PB1 is actuated toinitiate a test, the micro-controller 112 provides a sense test signalhaving an increasing pulse width at pin 10. As a result, themicro-controller 112 applies increasing voltage to the capacitor C2through the resistor R3 as the width of the pulse increases, therebycreating simulated electrical arcing at varying voltages.

As shown in FIG. 1 b, the firing circuit 108 includes capacitors C5-C7,resistors R11-R12, a diode D9, and a silicon controlled rectifier(SCR1). Specifically, the capacitor C7 is connected between the anodeand the cathode of the SCR1, and the capacitor C6 and the resistor R12are connected between the gate and the cathode of the SCR1 to prevent anunintentional turn-on of the SCR1 by a significant dv/dt event. Thecapacitor C5 is serially connected between pin 14 of themicro-controller 112 and the current limiting resistor R11 to preventexcessive depletion of the power supply. The electromechanical interface117 includes a diode bridge including diodes D12-D15, a solenoid 118,and a metal oxide varistor (MOV1) connected between the line neutral andline phase terminals to prevent excessive line voltage. The diode D9 isconnected between the diode bridge D12-D15 and the anode of the SCR1.The diode D9 isolates the capacitor C7 from a line voltage monitoringcircuit including the resistors R16-R17 and the capacitor C8 connectedto pin 8 of the micro-controller 112. Accordingly, when the SCR1 isturned on, the SCR1 draws increased current through the diode bridgeD12-D15, and a voltage level approximately equal to the line voltagetrips the solenoid 118 to disconnect the Power Output from the load.

In the illustrated embodiment, the power supply 106 includes resistorsR13-R20, capacitors C8-C10, and diodes D10-D11. The serially connectedresistors R13-R15 limit the amount of current provided to the zenerdiode D11. As shown in FIG. 1 b, the resistor R20 is connected betweenthe junction of the diode D10 and the zener diode D11 and the positivesupply Vcc (pin 2) of the micro-controller 112. The diode D10 preventsreverse current flow from the capacitor C9, which is connected betweenthe junction of the diodes D10-D11 and ground. Further, the capacitorC10, which is connected between pin 2 of the micro-controller 112 andground, provides the voltage Vcc to the micro-controller 112. Theserially connected resistors R16-R17 are connected between the junctionof the resistors R14-R15 and ground. Further, the capacitor C8, which isconnected between the junction of the resistors R16-R17 and ground,provides a reference voltage (VREF) to pin 8 of the micro-controller112. The reference voltage VREF is proportional to the voltage of thediode bridge D12-D15, which is approximately equal to the absolute valueof the line voltage. In the presently disclosed embodiment, themicro-controller 112 monitors the line voltage via VREF, and determineswhen to perform measurements of the voltage across the capacitor C3based on the monitored line voltage. In an alternative embodiment, themicro-controller 112 may monitor the output of a digital timer andperform voltage measurements across the integrating capacitor C3 basedon the timer output.

As described above, the micro-controller 112 determines the occurrenceof arcing events by processing stored voltage/pulse data using one ormore algorithms. A method of operating the arc fault detection apparatus100 that employs a three cycle algorithm (TCA) to reduce the occurrenceof tripping on nuisance loads is described below with reference to FIGS.1 b and 4. As depicted in step 402, the integrating capacitor C3 isreset to 0 volts and all flags within the micro-controller 112 areinitialized. Steps 406-409 form a subroutine in which the methodcontinues to loop until the until the reference voltage VREF monitoredat pin 8 of the micro-controller 112 exceeds a predetermined valuesamp_hi and then goes below a selected value sample1, thereby defining asampling or measurement point near the line voltage zero crossing. Ameasurement of the voltage across the capacitor C3 is then made, asdepicted in step 414, at pin 9 of the micro-controller 112, after whichthe capacitor C3 is reset to 0 volts. Next, a determination is made, asdepicted in step 416, as to whether the pushbutton PB1 is actuated. Inthe event the pushbutton PB1 is actuated, electrical arc-like noise isinjected, as depicted in step 420, into the capacitor C2 through theresistor R3 connected to pin 10 of the micro-controller 112 which, withsufficient noise injected over a plurality of half cycles of the linevoltage and processed by the TCA in step 440 described below, causes thesolenoid 118 to trip in the same manner as a detected electrical arc inthe load current.

In the preferred embodiment, because a minimal voltage is required tosustain an electrical arc, e.g., about 15 volts, a window typically upto 50 volts is selected for voltage measurement to account for phasedifferences between the line current and the line voltage. This windowaround the line voltage zero crossing captures relatively small arcsthat are typically generated or extinguished near the zero crossing.

Next, the voltage measurement is converted to digital form and pushed,as depicted in step 434, onto a stack within the micro-controller 112 tomaintain a history of measurement data. In the presently disclosedembodiment, successive voltage measurement values are entered as wordsinto the stack. The TCA is then executed, as depicted in step 440.Specifically, the word of cycle 1 (i.e., V[n−1]) minus the word of cycle2 (i.e., V[n]) is calculated and the absolute value is taken to obtain afirst calculated value, the word of cycle 3 (i.e., V[n+1]) minus theword of cycle 2 (i.e., V[n]) is calculated and the absolute value istaken to obtain a second calculated value, and the word of cycle 3(i.e., V[n+1]) minus the word of cycle 1 (i.e., V[n−1]) is calculatedand the absolute value is taken to obtain a third calculated value. Thefirst value plus the second value minus the third value is thencalculated and the absolute value is taken. The TCA executed in step 440may therefore be expressed asTCA=|(|V _([n−1]) −V _([n]) |+|V _([n+1]) −V _([n]) |−|V _([n+1]) −V_([n−1])|)|.  (3)It is understood that the outermost pair of absolute value signs inequation (3) are not strictly required, but are included to emphasizethe need to avoid the occurrence of least significant bit (LSB) errorsduring execution of the TCA. It is noted that the adjacent full cycles1-3 employed in the TCA may or may not be overlapping. If the threecycles are not overlapping, then six half cycles are required to executethe TCA. If the three cycles are overlapping, then only four half cyclesare required for the TCA.

Next, at least one continuous running sum of TCA calculations ismaintained, as depicted in step 444. Each running sum of TCAcalculations represents the total amount of electrical arcing thatoccurs over a respective sampling period. At the end of the samplingperiod, a determination is made, as depicted in step 446, as to whetherthe running sum exceeds a predetermined maximum threshold valuemax_limit. In the event the running sum value exceeds max_limit, an arcfault is detected and the SCR1 is fired, as depicted in step 448, todisconnect the Power Output from the load. In the preferred embodiment,the SCR1 is fired three times to assure firing even if there is a briefinterruption of the line voltage. In the presently disclosed embodiment,a pulse having a selected width, e.g., 30 μsec, is provided to the SCR1.The method then loops back to step 402 to prepare the integratingcapacitor C3 for subsequent voltage measurements.

Having described the above illustrative embodiments, other alternativeembodiments or variations may be made. For example, it was describedthat the arc fault detection apparatus 100 is implemented on asubstrate, and that the primary coil L1 of the transformer TR1 surroundsan inductive pickup coil having a magnetic axis that is perpendicular tothe substrate. FIG. 2 a depicts a first alternative embodiment, in whichthe primary coil L1 is implemented as a first U-shaped conductive trace212 disposed in and parallel to a substrate 202. In this embodiment, themagnetic axis of the inductive pickup coil is parallel to the substrate202, and the current flowing through the primary coil L1 isperpendicular to the axis of the pickup coil. As shown in FIG. 2 a, anelectrostatic shield comprising a second conductive trace 214 isdisposed between the first trace and the pickup coils. It is noted thatthe transformer TR1 may be implemented as a single surface mount pickupcoil, or as two surface mount pickup coils TR2-TR3 to provide a groundedcenter tap for reducing capacitive coupling between the primary andpickup coils. In the presently disclosed embodiment, the polarity of thetwo pickup coils TR2-TR3 is independent of 180° rotation about an axisperpendicular to the magnetic axis. In addition, the substrate 202functions as an insulator between the primary coil 212 and the pickupcoils.

As shown in FIG. 2 a, the two surface mount pickup coils TR2-TR3 aredisposed on one side of the substrate 202, and the primary coil trace212 is underneath and perpendicular to the magnetic axes of the pickupcoils. It is appreciated that the U-shaped trace 212 of the primary coilL1, which is connected to the line current, may be replaced by one linecurrent trace and one return current trace. In this way, arc detectioncapability may be provided even if the line and return connections arereversed.

FIG. 2 b depicts a second alternative embodiment, in which the twosurface mount pickup coils TR2-TR3 are placed substantially opposite oneanother on opposing sides of the substrate 202, and the primary coiltrace 212 is disposed between the pickup coils, thereby providingincreased mutual inductance. For example, each pickup coil may comprisea 5315TC (Ferrite) series RFID transponder coil sold by CoilcraftCorporation, Cary, Ill., USA, or any other suitable pickup coil.

It was also described that the input sense circuit 102 (see FIG. 1 b)may be configured to allow the micro-controller 112 to perform voltagemeasurements once per half cycle of the line voltage. FIG. 3 a depicts asum of logarithm circuit 102 a, which is a first alternative embodimentof the input sense circuit 102. Like the input sense circuit 102, thesum of logarithm circuit 102 a allows the micro-controller 112 tomeasure the voltage across the capacitor C3 once per half cycle near theline voltage zero crossing. As shown in FIG. 3 a, the sum of logarithmcircuit 102 a includes the capacitor C1, capacitors C12-C13, resistorsR23-R26, diodes D14-D15, and transistors T1-T4. The transistors T1-T4and the capacitor C13 form a logging circuit, and the change in voltageacross the capacitor C13 may be expressed asΔV _(C13) ≈νT*ln(|Q|/(C*νT)+1),  (4)in which “|Q|” is equal to the arc-produced charge flowing through theprimary coil L1 of the transformer TR1, andνT=kT/q,  (5)in which “vT” is equal to about 26 mV at room temperature. In thepresently disclosed embodiment, each arcing event increases the voltageacross the capacitor C2 by (C13/C2)*ΔV. Further, the voltage across thecapacitor C13 resets with a decay time of about (C13)*(R23) seconds. Forexample, if each one of the resistors R23-R26 equals 10 kΩ and thecapacitor C13 equals 10 nf, then the decay time is about 100 μsec.Accordingly, the charge injected by an arcing event into the capacitorC1 equals the charge injected into the capacitor C13, beginning at thetime of the arcing event and ending about a decay time later.

FIG. 3 b depicts a sum of logarithm circuit 102 b, which is a secondalternative embodiment of the input sense circuit 102 (see FIG. 1 b). Asshown in FIG. 3 b, the sum of logarithm circuit 102 b includes thecapacitor C1, resistors R27-R29, capacitors C14-C16, transistors T5-T6,and diodes D16-D19. In this alternative embodiment, the sum of logarithmcircuit 102 b employs only one matched transistor pair T5-T6 in thelogging circuit.

It was also described that the arcing sense circuit 104 (see FIG. 1 b)includes the op amp 116 and the resistors R5-R6, which form anon-inverting amplifier 105. FIG. 3 c depicts a comparator circuit 105 athat may be employed in place of the non-inverting amplifier 105. Asshown in FIG. 3 c, the comparator circuit 105 a includes a comparator120, resistors R30-R32, the diode D7, and the capacitor C3.Specifically, the resistors R30-R31 form a voltage divider that biasesthe inverting input of the comparator. When the level of the signalprovided to the non-inverting input of the comparator 120 by the inputsense circuit 102 exceeds the level at the inverting input of thecomparator 120, the comparator 120 charges the capacitor C3 through theresistor R32 at a rate proportional to (R32)*(C3). It is noted that thecapacitor C3 continues to charge the capacitor C3 so long as the signallevel at the non-inverting input of the comparator 120 remains greaterthan Vcc*[R31/R30+R31)]. Accordingly, each time a significant change inload current is detected at the input of the comparator circuit 105 a(i.e., each time a significant di/dt event occurs), the output of thecomparator is driven to its positive rail, thereby generating a pulsefor charging the capacitor C3 through the diode D7 and the resistor R7.

In the presently disclosed embodiment, the micro-controller 112 isoperative to execute a first pulse counter algorithm to count the numberof times the output of the comparator circuit 105 a (or thenon-inverting amplifier 105) is driven high during each half cycle. Dueto the generally chaotic nature of electrical arcing, arc faultstypically produce varying numbers of arcing events per half cycle of theline voltage. In contrast, nuisance loads typically produce the samenumber of arcing events per half cycle, and may therefore produce arcingevents periodically over multiple half cycles. Such information may beused to inhibit nuisance tripping under normal operating conditions, andto allow tripping to occur when arc faults are detected. Specifically,the comparator circuit 105 a provides the pulse counter signal to pin 13of the micro-controller 112, which uses this signal during the executionof the first pulse counter algorithm. Each time the output of thecomparator circuit 105 a is driven high during each half cycle, asindicated by the level of the pulse counter signal, a digital counterwithin the micro-controller 112 is incremented. When the capacitor C3 isreset by the micro-controller 112, the counter value is stored withinthe micro-controller 112, and the first pulse counter algorithm isexecuted. In the presently disclosed embodiment, the micro-controller112 executes the first pulse counter algorithm to determine theperiodicity of a predetermined number of data elements in one or moremeasurement data sets stored in the micro-controller 112. For example,in the event the stored counter value is equal to four, the first pulsecounter algorithm may be used to determine the periodicity of 1-4 dataelements in at least one measurement data set.

The operation of the first pulse counter algorithm is described belowwith reference to FIG. 5 a. As depicted in step 502, the pulse counteralgorithm searches for a predetermined number of data elements in a setof measurement data having the same first data value k. The entiremeasurement data set is then analyzed, as depicted in step 504, todetermine the presence of data elements having a value of zero (0's), todetermine the presence of additional data elements having the first datavalue (k's), and to determine the presence of data elements having asecond data value different from the first data value (j's) Next, thedata set is mapped, as depicted in step 506. For example, an exemplarydata set may contain a first data element having a zero value, a seconddata element having a first value of 3, a third data element having thesame first value of 3, and a fourth data element having a differentsecond value of 2, and may therefore be mapped to [0,k,k,j]. Next, adetermination is made, as depicted in step 508, as to whether the mappeddata set matches at least one of a plurality of predetermined data setsindicative of the periodicity of multiple arcing events. As explainedabove, nuisance loads typically produce periodic arcing events, whilearcing events produced by arc faults are typically non-periodic. Inaddition, certain start-up and shutdown conditions may resemble periodicarcing events. FIG. 6 depicts an illustrative mapping of a plurality ofdata sets indicative of periodic arcing events. For example, theexemplary mapping [0,k,k,j] described above does not match any of thedata sets shown in FIG. 6. In this case, the pulse counter is notregarded as being “active” (N) and tripping is allowed. In the event amatch is made, as depicted in step 510, the pulse counter is regarded asbeing “active” (Y) and tripping is inhibited, thereby reducing theoccurrence of nuisance tripping under normal operating conditions. Forexample, tripping may be inhibited by increasing the predeterminedmaximum threshold value max_limit and/or any other suitable constantand/or coefficient employed by the above-described three cycle algorithm(TCA). It is understood that the constants/coefficients employed, by theTCA may also be suitably modified to enable tripping when significantarc faults are detected.

As described above, the first pulse counter algorithm includes the stepof determining whether a mapped data set matches at least onepredetermined data set indicative of the periodicity of an arcing event.In an alternative embodiment, a determination may also be made as towhether the mapped data set matches one or more predetermined data setsindicative of insignificant events (e.g., noise) in the data history.For example, such a data set may map to [0,k,k,j,k,k], which would beindicative of periodicity but for the single “j” element in the mapping.By inhibiting tripping when such a match occurs, a degree of noisefiltering may be incorporated into the first pulse counter algorithm.

In the presently disclosed embodiment, a second pulse counter algorithmis also executed to capture timing information relating to one or morearcing events. The second pulse counter algorithm is described belowwith reference to FIG. 5 b. As depicted in step 514, the counter withinthe micro-controller 112 is employed to track the amount of timerequired for an arcing event to occur within the sampling period. Forexample, the counter may be used to measure the time from the beginningof the sampling period to the occurrence of an arcing event within thesampling period. A plurality of measured time values is then stored, asdepicted in step 516, over a number of sampling periods to provide ahistory of arcing event times. Next, the time history is analyzed, asdepicted in step 518, to determine the randomness of the arcing times. Adetermination is then made, as depicted in step 520, as to whether anarcing event occurs at substantially the same time during each samplingperiod. In the event the arcing occurs at substantially the same timeduring each sampling period, the arcing is deemed to be caused by anuisance load and tripping is inhibited, as depicted in step 522. It isunderstood that the constants/coefficients employed by theabove-described three cycle algorithm may be suitably modified toinhibit and/or enable tripping based on the arc timing history obtainedby the second pulse counter algorithm.

To account for the non-linearity of electrical arcing signals, thecomparator circuit 105 a may be modified to provide multiple digitaloutput signals corresponding to respective voltage threshold values.FIG. 3 d depicts an illustrative embodiment of a comparator circuit 105b providing multiple digital outputs. As shown in FIG. 3 d, thecomparator circuit 105 b includes comparators 121 and 123, and resistorsR33, R35, R37, and R39. Further, the non-inverting inputs of thecomparators 121 and 123 are connectable to the node 114 (see FIG. 1 b),and the outputs of the comparators 121 and 123 are connectable tosuitable digital inputs of the micro-controller 112. Specifically, theresistors R33, R35, and R37, R39 form voltage dividers that bias theinverting inputs of the comparators 121 and 123, respectively. Forexample, the voltage dividers may bias the comparators 121 and 123 atpredetermined low and high voltage levels, respectively. In thisalternative embodiment, the digital counter within the micro-controller112 starts to increment when the comparator 121 provides a first digitaloutput indicating that the level of the arcing signal has exceeded thelow voltage threshold. Next, the rate at which the counter isincremented is increased as the arcing level exceeds the high voltagethreshold, as indicated by a second digital output provided by thecomparator 123. The rate at which the counter is incremented is thendecreased as the arcing level goes below the successive thresholds, asindicated by the first and second digital outputs provided by therespective comparators 121 and 123.

It was also described that the TCA may be expressed asTCA=|(|V _([n−1]) −V _([n]) |+|V _([n+1]) −V _([n]) |−|V _([n+1]) −V_([n−1])|)|(see equation (3)). However, equation (3) provides a relatively smoothresponse to single arcing events. To achieve a response to single arcingevents that is more characteristic of an impulse, a modified TCA may beexpressed asKnob _(—) TCA=TCA _(—)1+(knob)*TCA _(—)2,  (6)in which “TCA_1” is expressed as equation (3), “knob” is a constant, and“TCA_2” is expressed asTCA _(—)2=|V _([n−1])−2*V _([n]) +V _([n+1])|,  (7)in which V[n−1] represents a first voltage measurement corresponding toa first cycle of the line voltage, V[n] represents a second voltagemeasurement corresponding to a second cycle of the line voltage, andV[n+1] represents a third voltage measurement corresponding to a thirdcycle of the line voltage. It is noted that TCA_2 provides more of animpulse response to single arcing events. In equation (6) above, theknob constant may be adjusted (e.g., the knob constant may be set to ⅛or any other suitable value) to provide varying amounts of impulseresponse.

It was also described that the resulting sum of the three cyclealgorithm (TCA) is added to a continuous running sum of TCA calculationsthat represents the total amount of electrical arcing occurring over thesampling period. At the end of each sampling period, the running sum iscompared to the predetermined maximum threshold value max_limit, and theSCR1 is fired in the event the threshold is exceeded. In an alternativeembodiment, to further avoid nuisance tripping, the micro-controller 112(see FIG. 1) is operative to execute an arcing event counter algorithmto count the number of arcing events included in the running sum.

The arcing event counter algorithm is described below with reference toFIG. 7. As depicted in step 702, the voltage across the capacitor C3 ismeasured. Next, a determination is made, as depicted in step 704, as towhether the measured voltage value exceeds a first predeterminedthreshold value. If the measured voltage value exceeds the firstthreshold value, then a first event counter within the micro-controller112 is incremented, as depicted in step 706. Next, at least one seconddetermination is optionally made, as depicted in step 708, as to whetherthe measured voltage value exceeds a second predetermined thresholdvalue. If the measured voltage value exceeds the second threshold value,then a second event counter within the micro-controller 112 isincremented, as depicted in step 710. Next, the measurement of thevoltage across the capacitor C3 is processed, as depicted in step 712,by at least one algorithm such as the TCA described above. In thepreferred embodiment, the voltage measurement is then added to first andsecond running sums of voltage measurements. For example, the firstrunning sum of voltage measurements may correspond to a short samplingperiod during which large voltage measurements are monitored over ashort period of time, and the second running sum of voltage measurementsmay correspond to a long sampling period during which smaller voltagemeasurements are monitored over a longer period of time. As depicted instep 714, a determination is made as to whether the first running sum(running sum 1) exceeds a first predetermined trip threshold (tripthreshold 1). In the event the first running sum value exceeds the firsttrip threshold value, a determination is made, as depicted in step 716,as to whether the output of the first event counter (event counter 1)exceeds a first predetermined minimum number of events (min. events 1).In the event the first event counter output exceeds the first number ofevents, tripping occurs, as depicted in step 722, to disconnect thePower Output from the load. As depicted in step 718, a determination ismade as to whether the second running sum (running sum 2) exceeds asecond predetermined trip threshold (trip threshold 2). In the event thesecond running sum value exceeds the second trip threshold value, adetermination is made, as depicted in step 720, as to whether the outputof the second event counter (event counter 2) exceeds a secondpredetermined minimum number of events (min. events 2). In the event thesecond event counter output exceeds the second number of events,tripping occurs, as depicted in step 722, to disconnect the Power Outputfrom the load. Accordingly, if either the first event counter outputexceeds the first predetermined count number or the second event counteroutput exceeds the second predetermined count number, then trippingoccurs. Otherwise, no tripping takes place.

In this way, nuisance tripping due to, e.g., noisy switching signals isavoided. Although such noisy signals may result in relatively largevoltage measurements, they are not necessarily indicative of electricalarcing. By monitoring the level of the running sum of measured voltages,and by tracking the number of arcing events included in the running sum,electrical arcing containing several half cycles of arcing events can bemore reliably detected, and nuisance loads containing only a limitednumber of arcing events can be more safely ignored.

It was also described that the micro-controller 112 monitors the linevoltage via VREF, and determines when to perform measurements of thevoltage across the capacitor C3 based on the monitored line voltage.Under normal operating conditions, the time between these voltagemeasurements is regular and periodic. However, during high currentarcing conditions, the VREF signal can become degraded due to a linevoltage drop out caused by a momentary hard short circuit. If themicro-controller 112 is looking for a specific voltage point on the halfcycle of the line voltage, then such a voltage drop out could cause aninadvertent or early command to measure. In addition, during this typeof arcing event, the voltage on the integrating capacitor C3 istypically relatively high. During a normal line drop out (or brown out),a purported measurement point may be found by the micro-controller, butthere may be no abnormal voltage on the capacitor C3. In contrast,during a high current arcing condition, a purported measurement pointmay be found and a relatively high voltage may be detected on thecapacitor C3. Accordingly, to detect high level arcing, themicro-controller 112 is operative to measure the time betweenmeasurement points. In the event an early measurement is found and arelatively large capacitor voltage V_(C3) is detected, themicro-controller activates the firing circuit 108, thereby tripping thesolenoid 118 to disconnect the Power Output from the load.

It was also described that the micro-controller 112 (see FIG. 1) maytake measurements of the voltage across the capacitor C3 twice each halfcycle, convert the voltage measurements to digital form using an analogto digital converter (ADC), store the measured voltage data, anddischarge the capacitor C3 when the voltage measurements are completed.It is noted that the voltage across the capacitor C3 represents theintegral of the signal provided by the non-inverting amplifier 105. Inan alternative embodiment, a digital counter within the micro-controller112 is employed as an accumulator for effectively integrating the outputof the non-inverting amplifier 105, thereby obviating the need for theintegrating capacitor C3 and the ADC.

In this alternative embodiment, the comparator circuit 105 a or 105 b isused in place of the non-inverting amplifier 105, and the digitaloutput(s) of the comparator circuit is provided directly to themicro-controller 112 for incrementing the internal counter. Further, thesampling period is defined as having a duration corresponding to one ormore half cycles of the line voltage. The operation of themicro-controller 112 and the internal counter for integrating the outputof the comparator circuit 105 a is described below with reference toFIG. 8. As depicted in step 802, the counter is reset at the beginningof the sampling period. Next, a determination is made, as depicted instep 804, as to whether the output of the comparator circuit 105 a isactive, thereby indicating the presence of electrical arcing. In theevent the comparator circuit output is active, the counter is started,as depicted in step 806. A determination is then made, as depicted instep 808, as to whether the output of the comparator circuit 105 a hasbecome inactive, indicating the termination of electrical arcing. In theevent electrical arcing has terminated, the method branches to step 812.Otherwise, a determination is made, as depicted in step 810, as towhether the end of the sampling period has been reached. In the eventthe end of the sampling period has been reached, the method proceeds tostep 812. Otherwise, the method loops back to step 808. Next, the outputvalue of the counter is stored, as depicted in step 812. The storedcounter output, which represents the integral of the output of thecomparator circuit 105 a, is subsequently used in place of the voltagemeasurements across the integrating capacitor C3 in the arc detectionalgorithms described above.

A method of operating the arc fault detection apparatus 100 that employsthe arc detection algorithms including the pulse counter algorithm, thethree cycle algorithm (TCA), and the arcing event counter algorithm isillustrated by reference to FIGS. 1 b and 9. As depicted in step 902,the integrating capacitor C3 is reset to 0 volts. Next, a determinationis made, as depicted in step 904, as to whether an early command tomeasure is detected. For example, the micro-controller 112 may detectsuch an early command to perform a voltage measurement if the VREFsignal becomes degraded due to a line voltage drop out. In the event anearly measurement is detected and an excessively large voltage acrossthe capacitor C3 is measured, an arc fault is detected, as depicted instep 924, and the solenoid 118 is tripped to disconnect the Power Outputfrom the load. Otherwise, the micro-controller 112 monitors the VREFsignal and waits, as depicted in step 906, for the sampling ormeasurement point to arrive.

When the measurement point arrives, the voltage across the integratingcapacitor C3 is measured, as depicted in step 908, after which thecapacitor C3 is reset to 0 volts. Next, the micro-controller 112monitors the pulse counter signal provided by the arcing sense circuit104 and stores, as depicted in step 910, the number of pulse countsoccurring during the sampling period. In addition, the micro-controller112 stores, as depicted in step 912, the integrating capacitor voltagemeasurement into the history data store (e.g., a stack). The storedpulse count information is then analyzed, as depicted in step 914, usingthe above-described pulse counter algorithm. Next, the three cyclealgorithm (TCA) is executed, as depicted in step 916, using the storedhistory of voltage measurement data, and resulting TCA calculations areadded to at least one running sum. One or more event counters within themicro-controller 112 are then adjusted, as depicted in step 918, and thevoltage measurements are analyzed using the above-described arcing eventcounter algorithm. A determination is then made, as depicted in step920, as to whether one or more event counters exceed predeterminedminimum numbers of arcing events. If the event counter(s) exceeds thepredetermined minimum number(s) of arcing events, another determinationis made, as depicted in step 922, as to whether the TCA running sum(s)exceeds a predetermined trip threshold value. In the event the TCArunning sum(s) exceeds the predetermined trip threshold, an arc fault isdetected, as depicted in step 924, and the solenoid 118 is tripped todisconnect the Power Output from the load.

It should be appreciated that the above-described arc fault detectionapparatus 100 may also be employed to provide DC arc fault detection. Inthis case, instead of monitoring the line voltage via VREF to determinewhen to perform voltage measurements, the micro-controller 112 mayalternatively employ an internal digital counter as a time base togenerate suitable periods for sampling the voltage across theintegrating capacitor C3.

It will further be appreciated by those of ordinary skill in the artthat further modifications to and variations of the above-describedapparatus and method for detecting arc faults may be made withoutdeparting from the inventive concepts disclosed herein. Accordingly, theinvention should not be viewed as limited except as by the scope andspirit of the appended claims.

1. A method of detecting arc faults, comprising the steps of: sensing asignal associated with a power input by an input sense circuit, thesensed signal being indicative of at least one potential arcing event;receiving a representation of the sensed signal by an amplifyingcircuit; generating a respective pulse corresponding to the at least onepotential arcing event by the amplifying circuit; receiving therespective pulses by an accumulating circuit; generating an accumulatedsignal over a predetermined time period by the accumulating circuit;receiving the accumulated signal and the respective pulses by aprocessor; generating first data representative of the accumulatedsignal and second data representative of the respective pulses by theprocessor; and processing at least one of the first and second data inaccordance with at least one algorithm by the processor, therebydetermining whether the at least one potential arcing event isindicative of an arc fault or a nuisance condition.
 2. The method ofclaim 1 further including the steps of monitoring the power input by acurrent sensor, and providing the signal associated with the power inputto the input sense circuit by the current sensor.
 3. The method of claim2 wherein the current sensor comprises a transformer having a primarycoil and a secondary coil, the secondary coil being weakly coupled tothe input sense circuit.
 4. The method of claim 1 further including thestep of tripping an electromechanical interface coupled between thepower input and a power output if the at least one potential arcingevent is indicative of an arc fault, thereby disconnecting the poweroutput from a load coupleable thereto.
 5. The method of claim 4 whereinthe electromechanical interface comprises a solenoid.
 6. The method ofclaim 1 wherein the amplifying circuit comprises a comparator.
 7. Themethod of claim 1 wherein the accumulating circuit comprises anintegrating capacitor.
 8. The method of claim 1 wherein the accumulatingcircuit comprises a digital counter.
 9. The method of claim 1 whereinthe amplifying circuit comprises a plurality of comparators, and furtherincluding the steps of generating a respective pulse by a respective oneof the comparators in the event the sensed signal exceeds a respectivepredetermined threshold, receiving the respective pulses generated bythe plurality of comparators by the processor, and incrementing adigital counter at different rates based on the received pulses by theprocessor.
 10. The method of claim 1 further including the steps ofmeasuring the accumulated signal a number of times over a set of cyclesby the processor, generating the first data representative of theaccumulated signal measurements by the processor, and processing thefirst data in accordance with an algorithm including the sub-steps ofcalculating a first measurement associated with a first cycle minus asecond measurement associated with a second cycle to obtain a firstcalculated value, calculating a third measurement associated with athird cycle minus the second measurement associated with the secondcycle to obtain a second calculated value, calculating the thirdmeasurement associated with the third cycle minus the first measurementassociated with the first cycle to obtain a third calculated value,taking the absolute values of the first, second, and third calculatedvalues, and calculating the first value plus the second value minus thethird value to obtain a fourth calculated value.
 11. The method of claim10 further including the steps of repeating the processing of the firstdata in accordance with the algorithm using signal measurementscorresponding to at least one different set of cycles, and maintaining arunning sum of the fourth calculated values, the running sum beingrepresentative of the total amount of electrical arcing occurring duringthe predetermined time period.
 12. The method of claim 11 furtherincluding the step of processing the second data in accordance with atleast one second algorithm to count the number of arcing events includedin the running sum.
 13. The method of claim 1 further including the stepof processing the second data in accordance with an algorithm forcounting the number of arcing events occurring during the predeterminedtime period by the processor.
 14. The method of claim 13 furtherincluding the step of capturing timing information relating to thearcing events.